Boris Johnson has been urged to take foreign governments through the international courts to make them pay London's congestion charge after they racked up a bill topping £105m.
Transport for London has written to the foreign secretary, a former London mayor, to help end the 13 year impasse.
In total 145 nations have outstanding charges. The worst offenders include the US, Japan, Nigeria and Russia.
The Foreign Office insists diplomats are not exempt from paying the charge.
According to Transport for London (TfL), around three quarters of embassies in London do pay the charge, "but there remains a stubborn minority who refuse to do so, despite our representations through diplomatic channels".
The US Embassy, which argues it is a tax and so covered by diplomatic immunity, owes the most, at more than £11.5m, while the Australian High Commission is among nations that owe the least, at £120.
Top 10 congestion charge embassy debtors:
- American Embassy - £11,544,455.00
- Embassy of Japan - £7,629,370.00
- High Commission for the Federal Republic of Nigeria - £6,481,620.00
- Embassy of the Russian Federation - £5,603,320.00
- Office of the High Commissioner for India - £4,991,125.00
- Embassy of the Federal Republic of Germany - £4,221,590.00
- Embassy of the Republic of Poland - £3,854,130.00
- Embassy of the People's Republic of China - £3,805,465.00
- Office of the High Commissioner for Ghana - £3,465,960.00
- The Embassy of the Republic of the Sudan - £3,048,475.00
Following legal advice, TfL's view is that the matter should be taken to the International Court of Justice in The Hague, but this needs government support, which is why it has been raised with the Foreign and Commonwealth Office.
Paul Cowperthwaite, TfL's general manager for congestion charging, said: "We are clear that the congestion charge is a charge for a service and not a tax. This means that foreign diplomats are not exempt from paying it.
"We continue to pursue all unpaid congestion charge fees and related penalty charge notices.
"In October we wrote to the foreign secretary to ask him to take up the matter with the relevant embassies and the International Court of Justice."
TfL figures suggest that London-based embassies owe £105,258,715 for the period since the congestion charge was launched in 2003 and 31 December 2016 - up from £58m in 2012.
A Foreign Office spokesman said: "Our position is clear: there are no legal grounds to exempt diplomats from paying the London congestion charge. This is not a tax, and the majority of diplomatic missions in London recognise this and pay.
"We are committed to working with TfL to find a solution to the problem of non-payment by some missions."
The congestion charge was introduced by former London mayor Ken Livingstone in February 2003, with the aim of reducing traffic congestion in and around the city centre between 07:00 and 18:00 Mondays to Fridays. The current charge is £11.50 per day.